260 BOTANY 



I'ART I 



From the fact that in different organs zones of different length 

 are in a growing condition, it follows that such results as to the total 

 growth of an organ as was described on p. 257 do not give the true rate 

 .of growth, i.e. the growth of a unit of length in unit time. Thus in the 

 shoots of the Bamboo the growing zone is many centimetres long, 

 while in Botrytis it is only 0'02 mm. in length. While Bamhusa 

 shows twice as much growth per minute than Botrytis does, its rate of 

 growth is really much les.s. To express the rapidity of growth it is 

 necessary to express the elongation per minute as a percentage of 

 the growing zone. This gives a rapidity of growth of 83 per 

 cent, in Botrytis, and of only 1"27 per cent in Bamhusa. The 

 maximum gi'owth observed is 220 per cent in some pollen tubes, 

 while some shoots which are still clearly growing have a rate of 

 only 0"5 per cent. 



AVe can only determine the definite elongation of a part of the plant 

 Avhen, in addition to the rate of growth and tiie length of the growing 

 region, the duration of growth is known. The size of the plant, which, 

 as is well known, depends in vai'ious ways on external conditions and 

 yet is a specific character, is determined by variations in these factors. 

 A definite size belongs to the specific properties of an organism just as 

 much as the form of its leaves, etc. ; further the whole organisation of 

 the plant is such that it involves a particular size. 



Hitherto our consideration has been limited to the expansion in 

 the longitudinal direction of parts laid down at the growing point. 

 Microscopic investigation shows, however, that a growth in thickness 

 also takes place. The diameter of the mature root or stem is greater, 

 often con.siderably greater, than the same diameter immediately 

 behind the growing point. This growth in thickness has been much 

 less thoroughly investigated than has the gi'owth in length, but all 

 essential processes which we ascertained regarding the giowth in 

 length are also found in this case. In particular a well-marked 

 grand period has been shown to exist. As has been shown on p. 138, 

 the primary growth in thickness has to be distinguished from 

 secondary growth in thickness. Only the primar}'- growth in 

 thickness is a process of the kind we are here considering ; secondary 

 growth is due to an intercalary meristem or cambium. 



The expansion of the cells in length and breadth does not always 

 take place uniformly and simultaneously in the whole cross section of 

 an organ. It is usual to find that, in giowing stems for instance, 

 the pith strives to expand more strongly than the peripheral tissues. 

 Since no breach of continuity between the two regions is possible a 

 state of tension (tissue tension) (''''^) results. The pith expands the 

 cortical ti.s.sues and these compress the pith ; the actual length of the 

 organ is the resultant of these antagonistic tendencies. If the tissues 

 are artificially separated, each assumes its own specific length ; the 

 pith elongates and the cortex contracts and the tension disaj)pears. 



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